Since recent wide-screen displays have improved brightness, they can be satisfactorily used in a brightly illuminated environment, and a demand for such displays is growing. Coordinate input apparatuses are increasingly required to be resistant against disturbance light for the purpose of use in combination with such wide-screen displays.
Recently, many devices use infrared light as a radio communication means. Since disturbance light tends to increase for both infrared and visible light, the resistance against disturbance light is one of important characteristics of an apparatus.
However, in an apparatus using a conventional CCD sensor disclosed in Japanese Patent Publication No. 7-76902 or Japanese Patent Laid-Open No. 6-274266, disturbance light can be suppressed only by an optical filter.
To the contrary, an apparatus using a PSD disclosed in Japanese Patent Application No. 2,503,182 exhibits strong resistance against disturbance light in cooperation with an optical filter because the apparatus can suppress the influence of disturbance light by frequency-modulating the light intensity and synchronously detecting the modulated wave.
A wide-screen display is increasing in resolution as well as in brightness. Hence, the resolving power of a coordinate input apparatus must also be improved, though the apparatus using a PSD resistant against disturbance light has a problem in this point.
That is, since the dynamic range of the sensor output voltage directly corresponds to the input range, an S/N ratio of at least 60 dB is required when the entire screen is segmented into, e.g., 1,000 coordinates. In addition, as described in Japanese Patent Application No. 2,503,182, since digital correction for a linear error is indispensable, a highly accurate analog circuit, a multi-bit A/D converter, and an arithmetic circuit are necessary. Furthermore, since the S/N ratio of a sensor output signal depends on the light amount and the sharpness of the light spot, suppressing disturbance light is insufficient, and a bright and accurate optical system is also required. These make the apparatus itself very expensive and bulky.
As a method of increasing the resolving power using a CCD, simultaneously using a plurality of video cameras is disclosed in Japanese Patent Publication No. 7-76902, though this method makes the apparatus bulky and expensive. An apparatus using a video camera with a lot of pixels becomes further bulky and expensive than that using a plurality of cameras. To achieve a resolving power more than the number of pixels, high-speed processing of an enormous quantity of image data is necessary. For real-time operation, the apparatus again becomes very bulky and expensive.
In Japanese Patent Laid-Open No. 6-274266, a high resolving power is obtained by a special optical mask and signal processing. If disturbance light is small, and a satisfactory S/N ratio can be ensured, the resolving power can be increased. In fact, since a linear sensor forms a linear image which cannot be separated from disturbance light in a plane, unlike an area sensor for forming a point image, and is therefore readily affected by disturbance light, the apparatus can be used only in a special environment with little disturbance light.
A scheme aiming at suppressing the influence of disturbance light and providing an inexpensive coordinate input apparatus is disclosed in Japanese Patent Laid-Open No. 11-219253, which blinks a light-emitting element incorporated in a writing tool and detects the difference signal between the signal in the ON state and that in the OFF state with a linear sensor to suppress the influence of disturbance light, and the position of the writing tool is detected on the basis of which pixel in the linear sensor receives the difference signal.
However, the coordinate input apparatus disclosed in Japanese Patent Laid-Open No. 11-219253 has the following problem. FIG. 7 shows the layout relationship between two linear sensors 20X and 20Y, the layout relationship between cylindrical lenses 90X and 90Y that serve as an imaging optical system, and a state wherein a light spot 5 on a coordinate input surface 10 forms linear images (images 91X and 91Y in FIG. 7) on photosensitive portions 21X and 21Y of the linear sensors 20X and 20Y through the cylindrical lenses 90X and 90Y.
When the two linear sensors 20X and 20Y are accurately laid out perpendicularly to each other, each linear sensor can obtain a sensor output signal with a maximum optical output on a pixel that reflects the X- or Y-coordinate of the light spot 5. When the images 91X and 91Y of the light spot 5 are appropriately blurred by focus adjustment, the image width becomes several times larger than that of a pixel of the linear sensors 20X and 20Y, and optical outputs are obtained from a plurality of pixels. By obtaining the barycentric position of the plurality of pixel signals, output pixel numbers αX and αY of the linear sensors 20X and 20Y corresponding to the light spot 5 can be calculated.
A conventional technique of calculating coordinate values using the output pixel numbers αX and αY will be described.
To calculate coordinates from the barycenter (αX,αY) of output data, the output pixel numbers αX and αY of the linear sensors 20X and 20Y corresponding to the light spot 5 at a predetermined known position must be stored. That is, when the output pixel numbers of the linear sensors 20X and 20Y corresponding to the light spot 5 at the first known point with coordinate values (XC,YC) and at the second known point with coordinate values (X1,Y1) are defined as (αX0,αY0) and (αX1,αY1) respectively, the coordinate values (X,Y) of the light spot 5 at an arbitrary position to be detected are given byX=(αX−αX0)(X1−X0)/(αX1−αX0)+X0  (1)Y=(αY−αY0)(Y1−Y0)/(αY1−αY0)+Y0  (2)
FIG. 17 is a graph showing the coordinate calculation accuracy of a coordinate input apparatus, which is obtained by the above calculations. Referring to FIG. 17, the X- and Y-axes represent the coordinate values of the coordinate input surface. The coordinate values (0,0) correspond to the central position of the coordinate input effective area. When optical elements such as lenses are laid out geometrically symmetrically with respect to the origin, the optical characteristics in the respective quadrants are symmetrical with respect to the origin. FIG. 17 shows a result obtained by checking the coordinate calculation accuracy of the coordinate input apparatus in a given quadrant.
FIGS. 14A and 14B are views showing the optical layout only in the X-axis direction (X-axis sensor 20X and cylindrical lens 90X). The X- and Y-axes in FIG. 17 are the coordinate value axes of the coordinate input surface and correspond to the X- and Y-axes in FIG. 14A. The Z-axis in FIG. 17 represents the difference between the coordinate values to be actually calculated and the resultant coordinate values of the coordinate input apparatus, i.e., the coordinate calculation accuracy of the coordinate input apparatus.
This analysis result is obtained by setting the known points (X0,Y0) and (X1,Y1) in equations (1) and (2), i.e., the first known point (X0,Y0) at the origin (the central position of the coordinate input effective area) and the second known point (X1,Y1) at the central position of the coordinate input effective area in that quadrant. According to FIG. 17, the accuracy gradually degrades in a region where the X-axis value becomes large and the Y-axis value also becomes large (the accuracy of this coordinate input apparatus is about 11 mm, as is apparent from the graph). Although FIG. 17 shows the coordinate calculation accuracy in the X-direction, that in the Y-direction also indicates the same result as described above.
That is, the coordinate calculation accuracy in the prior art degrades in the region where the X-axis value becomes large and the Y-axis value also becomes large. Analysis by numerical simulation reveals that the phenomenon occurs due to use of a cylindrical lens. To solve this distortion, a cylindrical lens must have, e.g., an aspherical surface not only in a direction perpendicular to the axis of the cylindrical lens but also along a section in a direction parallel to the axis. Even when such a lens can be optically designed, a plastic lens mold is hard to prepare, and the lens becomes very expensive.